Internet connectivity has transformed life everywhere as more people connect to the Internet to chat with friends and family, watch videos, listen to streamed music, or conduct online banking and e-commerce. The two primary means for access to the Internet are wired broadband and wireless. Current wired broadband Internet access is based on three different standards: Digital Subscriber Line (DSL); Data Over Cable Service Interface Specification (DOCSIS); and Fiber-to-Home (FTTH). The wireless access is based on two standards: Wide Area Network (WAN), also referred to as the Fourth Generation Long Term Evolution (4G LTE); and Local Area Network (LAN), also referred to as Wi-Fi. Wi-Fi is generally used indoors as short-range wireless extension of wired broadband systems. The 4G LTE on the other hand provides wide area long-range connectivity using dedicated infrastructure such as cell towers and backhaul to connect to the Internet.
In order to address the rapid growth in data traffic, next generation WLAN and cellular systems are expected to operate at higher frequencies where abundant spectrum is available. For example, at millimeter wave frequencies (28 GHz and above), radio spectrum use is lighter. A large number of small antennas operating at millimeter wave frequencies may be used to provide the increased capacity required in the future. The small size antennas are enabled by carrier waves that are millimeters long compared to centimeter long waves at currently used lower frequencies.
A number of wireless transceivers have been proposed for millimeter wave bands. The proposed wireless transceivers generally include multiple signal paths where each signal path may be connected to one or more antennas. A drawback of the proposed transceivers is that in transmit paths during up-conversion as digital signals are transformed into analog signals, their carrier frequencies are shifted. Similarly, in receive paths during down-conversion as analog signals are transformed into digital signals, their carrier frequencies are shifted. The shift in carrier frequency during up-conversion and down-conversion is generally referred to as a carrier frequency offset (CFO) which introduces unpredictability in the operation of the transceivers.
FIGS. 1 and 2 illustrate CFOs during up-conversion and down-conversion, respectively. For example, in FIG. 1 the kth transmitted analog baseband signal is ideally upconverted to frequency fk, ideal, but is actually upconverted to frequency fk=fk, ideal+fk, txoffset. If fk, txoffset is negative, fk, ideal shifts to a lower frequency, as demonstrated in FIG. 1. If fk, txoffset is positive, fk, ideal shifts to a higher frequency. Similarly, in FIG. 2 the kth received analog RF signal is ideally down-converted to frequency 0 Hz, but is actually down-converted to frequency fk, txoffset+fk, rxoffset.
Existing methods to correct or mitigate CFOs rely on the assumption that all signal paths have the same CFO value. However, in a wireless link featuring multiple signal paths each with different center frequencies, each signal path may have a different CFO value and, furthermore, the phase characteristics may be different in each signal. Therefore, a single CFO estimate for all signal paths may result in substantial residual CFO on each signal path after correction. Uncorrected CFOs cause progressive phase rotation of decoded complex baseband symbols. Consequently, the received error vector magnitude (EVM), which is a measure of received signal integrity, progressively degrades as the length of the data packets increase, thereby reducing packet lengths.